Date: Tue, 14 May 2024 14:48:01 +0200
From: Clément Léger <cleger@...osinc.com>
To: Conor Dooley <conor@...nel.org>
Cc: Conor Dooley <conor.dooley@...rochip.com>,
 Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Anup Patel <anup@...infault.org>, Shuah Khan <shuah@...nel.org>,
 Atish Patra <atishp@...shpatra.org>, linux-doc@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, kvm@...r.kernel.org,
 kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 02/11] riscv: add ISA extensions validation



On 14/05/2024 14:43, Conor Dooley wrote:
> On Tue, May 14, 2024 at 09:53:08AM +0200, Clément Léger wrote:
>>
>>
>> On 30/04/2024 13:44, Conor Dooley wrote:
>>> On Tue, Apr 30, 2024 at 09:18:47AM +0200, Clément Léger wrote:
>>>>
>>>>
>>>> On 30/04/2024 00:15, Conor Dooley wrote:
>>>>> On Mon, Apr 29, 2024 at 05:04:55PM +0200, Clément Léger wrote:
>>>>>> Since a few extensions (Zicbom/Zicboz) already needs validation and
>>>>>> future ones will need it as well (Zc*) add a validate() callback to
>>>>>> struct riscv_isa_ext_data. This require to rework the way extensions are
>>>>>> parsed and split it in two phases. First phase is isa string or isa
>>>>>> extension list parsing and consists in enabling all the extensions in a
>>>>>> temporary bitmask without any validation. The second step "resolves" the
>>>>>> final isa bitmap, handling potential missing dependencies. The mechanism
>>>>>> is quite simple and simply validate each extension described in the
>>>>>> temporary bitmap before enabling it in the final isa bitmap. validate()
>>>>>> callbacks can return either 0 for success, -EPROBEDEFER if extension
>>>>>> needs to be validated again at next loop. A previous ISA bitmap is kept
>>>>>> to avoid looping mutliple times if an extension dependencies are never
>>>>>> satisfied until we reach a stable state. In order to avoid any potential
>>>>>> infinite looping, allow looping a maximum of the number of extension we
>>>>>> handle. Zicboz and Zicbom extensions are modified to use this validation
>>>>>> mechanism.
>>>>>
>>>>> Your reply to my last review only talked about part of my comments,
>>>>> which is usually what you do when you're gonna implement the rest, but
>>>>> you haven't.
>>>>> I like the change you've made to shorten looping, but I'd at least like
>>>>> a response to why a split is not worth doing :)
>>>>
>>>> Hi Conor,
>>>>
>>>> Missed that point since I was feeling that my solution actually
>>>> addresses your concerns. Your argument was that there is no reason to
>>>> loop for Zicbom/Zicboz but that would also apply to Zcf in case we are
>>>> on RV64 as well (since zcf is not supported on RV64). So for Zcf, that
>>>> would lead to using both mecanism or additional ifdefery with little to
>>>> no added value since the current solution actually solves both cases:
>>>>
>>>> - We don't have any extra looping if all validation callback returns 0
>>>> (except the initial one on riscv_isa_ext, which is kind of unavoidable).
>>>> - Zicbom, Zicboz callbacks will be called only once (which was one of
>>>> your concern).
>>>>
>>>> Adding a second kind of callback for after loop validation would only
>>>> lead to a bunch of additional macros/ifdefery for extensions with
>>>> validate() callback, with validate_end() or with both (ie Zcf)). For
>>>> these reasons, I do not think there is a need for a separate mechanism
>>>> nor additional callback for such extensions except adding extra code
>>>> with no real added functionality.
>>>>
>>>> AFAIK, the platform driver probing mechanism works the same, the probe()
>>>> callback is actually called even if for some reason properties are
>>>> missing from nodes for platform devices and thus the probe() returns
>>>> -EINVAL or whatever.
>>>>
>>>> Hope this answers your question,
>>>
>>> Yeah, pretty much I am happy with just an "it's not worth doing it"
>>> response. Given it wasn't your first choice, I doubt you're overly happy
>>> with it either, but I really would like to avoid looping to closure to
>>> sort out dependencies - particularly on the boot CPU before we bring
>>> anyone else up, but if the code is now more proactive about breaking
>>> out, I suppose that'll have to do :)
>>> I kinda wish we didn't do this at all, but I think we've brought this
>>> upon ourselves via hwprobe. I'm still on the fence as to whether things
>>> that are implied need to be handled in this way. I think I'll bring this
>>> up tomorrow at the weekly call, because so far it's only been you and I
>>> discussing this really and it's a policy decision that hwprobe-ists
>>> should be involved in I think.
>>
>> Hi Conor,
>>
>> Were you able to discuss that topic ?
> 
> I realised last night that I'd not got back to this thread and meant to
> do that today (I had accidentally deleted it from my mailbox), but I had
> a migraine this morning and so didn't.
> I did bring it up and IIRC Palmer was of the opinion that we should try
> our best to infer extensions.
> 
>>> Implied extensions aside, I think we will eventually need this stuff
>>> anyway, for extensions that make no sense to consider if a config option
>>> for a dependency is disabled.
>>> From talking to Eric Biggers the other week about
>>> riscv_isa_extension_available() I'm of the opinion that we need to do
>>> better with that interface w.r.t. extension and config dependencies,
>>> and what seems like a good idea to me at the moment is putting tests for
>>> IS_ENABLED(RISCV_ISA_FOO) into these validate hooks.
>>>
>>> I'll try to look at the actual implementation here tomorrow.
>>
>> Did you found time to look at the implementation ?
> 
> No, with the above excuse. I'll try to get to it today or tomorrow...

No worries, I was on vacation and was just checking if I hadn't missed
anything in the meantime. Take your time ;)

Thanks,

Clément

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